1. Field of the Invention
This invention relates to an electrophotographic photosensitive member and a conducting member which are used in a process cartridge and an electrophotographic apparatus. More particularly, it relates to a conducting member which electrically controls contact-object members, such as electrophotographic photosensitive members, charging members, developer-carrying members, transfer members, cleaning members and charge-eliminating members, which are used in electrophotographic apparatus, such as printers, facsimile machines and copying machines and in process cartridges detachably mountable to these apparatus.
2. Related Background Art
Charging processes in electrophotographic processes have conventionally widely employed a corona charging assembly with which the surface of a charging object-member photosensitive member is uniformly charged to a stated polarity and potential by a corona shower generated by applying a high voltage (DC voltage of 6 to 8 kV) to a metal wire. However, there have been problems such as the requirement for a high-voltage power source and the generation of ozone in a relatively large quantity.
As a countermeasure thereto, a contact charging system, in which a voltage is applied while bringing a charging member into contact with a photosensitive member to charge the surface of the photosensitive member, has put into practical use. This is a system in which a roller type, blade type, brush type or magnetic brush type charging member, serving as an electric-charge feed member, is brought into contact with a photosensitive member and a stated charging bias is applied to this contact charging member to uniformly charge the photosensitive member surface to a stated polarity and potential.
This charging system has the advantages that power sources can be made low-voltage and the generation of ozone can be lessened. In particular, a roller charging system employing a conductive roller (charging roller) as the contact charging member is preferably used in view of the stability of charging. With regard to the uniformity of charging, however, it is a little disadvantageous over the corona charging assembly.
In order to improve charging uniformity, as disclosed in Japanese Patent Application Laid-Open No. 63-149669, an xe2x80x9cAC charging systemxe2x80x9d is used in which an alternating voltage component (AC voltage component) having a peak-to-peak voltage that is at least twice the charge-starting voltage (VTH) is superimposed on a DC voltage corresponding to the desired charging object surface potential Vd and a voltage thus formed (a pulsating voltage whose value changes periodically with time) is applied to the contact charging member. This system aims at the potential-leveling effect attributable to AC voltage. The potential of the charging object member converges on the potential Vd, which is the middle of the peak of AC voltage, and is not affected by any external disorder in the environment or the like. Thus, this is as good a method as the contact charging method.
Since, however, this method uses a high-voltage AC voltage having a peak-to-peak voltage that is at least twice the discharge-starting voltage (VTH) at the time of the application of DC voltage is superimposed, an AC power source is required in addition to a DC power source. This causes the apparatus itself to have a high cost. Moreover, since AC current is consumed in large quantities, there has been a problem that the running performance of the charging roller and the photosensitive member tends to decrease.
These problems can be solved by applying only DC voltage to the charging roller to effect charging. However, the application of only DC voltage to the charging roller has caused the following problems.
The application of only DC voltage to a conventional charging member causes, on the surface of the charging object member such as the photosensitive member, an uneven potential due to excessive charging beyond the desired charge potential (hereinafter xe2x80x9cexcessive-charging uneven potentialxe2x80x9d). In particular, in an electrophotographic process having no pre-exposure, which is a step for eliminating before primary charging the potential on the photosensitive member, such excessive-charging uneven potential tends to occur at potential portions of halftone image areas. Where the photosensitive-member surface potential at halftone potential portions is measured with a surface potentiometer, an uneven potential due to charging as excessive as about tens of volts in potential difference is observable at places corresponding to the second- and subsequent-round positions on the photosensitive member.
When halftone images are reproduced using a conventional charging roller causing such a problem, by means of, e.g., an electrophotographic apparatus employing a reversal development system, there has been a problem that the above excessive-charging uneven potential appears on images as partially blank or coarse halftone image areas, resulting in a low image quality. This excessive-charging uneven potential tends to especially remarkably occur in a low-temperature and low-humidity environment.
As a method in which only the DC voltage is applied to achieve a uniformity of charging, Japanese Patent Application Laid-Open No. 5-341626 discloses a technique in which an upstream-side microgap, formed between the charging member and the charging object member, is irradiated by light (nip exposure) to remove electric charges from the charging object-member surface, which is then charged via a downstream-side microgap. By this method, the charging object-member surface can be charged relatively uniformly, but not satisfactorily.
In the electrophotographic apparatus employing the contact charging system, uneven image density may also occur because of faulty charging due to contamination of the charging member (adhesion of developer to its surface), which tends to cause a problem with running performance. Accordingly, in order to enable many-sheet printing, it has been a pressing need to prevent the influence of faulty charging due to contamination of the charging member. Especially in the case of the DC charging system where only the DC voltage is applied to the charging member, the influence of contamination of the charging member tends to cause more faulty images than in the case of the AC charging system.
The present invention was made taking account of the foregoing. Accordingly, an object of the present invention is to provide a process cartridge, and an electrophotographic apparatus, which may hardly cause an excessive-charging uneven potential even when the charging object member is charged by applying only DC voltage to a conducting member.
Another object of the present invention is to provide a process cartridge, and an electrophotographic apparatus, which may hardly cause faulty charging due to contamination of a conducting member and can maintain a good charging performance over a long period of time.
To achieve the above objects, the present invention provides a process cartridge comprising an electrophotographic photosensitive member and a conducting member disposed in contact with the electrophotographic photosensitive member and to which a voltage is to be applied;
the electrophotographic photosensitive member and conducting member being supported as one unit and being detachably mountable to the main body of an electrophotographic apparatus;
the electrophotographic photosensitive member comprising a support, and a charge generation layer and a charge transport layer in this order provided on the support; the charge transport layer having a thickness of from 12 xcexcm to 40 xcexcm; and
the conducting member comprising a conductive support and a covering layer provided thereon; the time constant xcfx84 of electric current of the conducting member being 0.1 second or shorter.
The present invention also provides an electrophotographic apparatus comprising an electrophotographic photosensitive member and a conducting member disposed in contact with the electrophotographic photosensitive member and to which a voltage is to be applied;
the electrophotographic photosensitive member comprising a support, and a charge generation layer and a charge transport layer provided on the support in this order; the charge transport layer having a thickness of from 12 xcexcm to 40 xcexcm; and
the conducting member comprising a conductive support and a covering layer provided thereon; the time constant xcfx84 of electric current of the conducting member being 0.1 second or shorter.